A hydrometallurgical process for the treatment of nickel-cobalt-copper sulphide concentrates and mattes to produce high grade nickel and cobalt powders has been in commercial operation for many years. In this process, which is described in the publication, "The Winning of Nickel", J. R. Boldt, Jr. (Ed), Toronto, Longmans, Canada Limited, 1967, pages 299 to 314, the nickel sulphide feed materials are leached in ammoniacal ammonium sulphate solution, under elevated air pressure, in horizontal autoclaves. The metals, nickel, cobalt and copper are dissolved as metal ammine complex ions. Iron is oxidized to insoluble haematite, and sulphide sulphur is partially oxidized to form a range of unsaturated sulphur anions which remain in the leach solution. The haematite residue is filtered off, washed and discarded, and the leach solution is treated to remove copper, to partially remove the ammonia, and to oxidize the sulphur anions to sulphate. The purified solution, which contains about 60 g/L, Ni and 1 g/L Co, is then treated with hydrogen under elevated pressure and temperature to precipitate nickel selectively as the metal powder. The cobalt remains in the reduced solution which contains about 1 g/L Ni and 1 g/L Co. The residual metals are then precipitated using hydrogen sulphide to produce a mixed nickel cobalt sulphide which forms the feed to the cobalt refining process.
The recovery of cobalt, in this nickel refining process, is much lower than that of nickel, as a result of the adsorption of cobalt, by the iron oxide leach residue. Furthermore, the selectivity of the reduction process, for nickel, depends on the maintenance of a low level of cobalt in the purified leach solution. It is only possible to produce nickel powder meeting the market specifications for cobalt, if the Ni:Co ratio is maintained above about 20:1. It will be appreciated, therefore, that this nickel refining process is not economically effective for the treatment of nickel feed materials with high cobalt content, since significant losses of cobalt to the leach residue and to the nickel powder product will be incurred.
An earlier commercial cobalt refining process based on the soluble cobaltic pentammine process for separating nickel from cobalt, had been operated in conjunction with the above nickel refining process. This process was based on U.S. Pat. Nos. 2,694,005; 2,694,006; 2,767,054 and 2,767,055 to Schaufelburger.
When limonitic nickel laterite ores are treated by high temperature sulphuric acid pressure leaching, the dissolved nickel and cobalt are both recovered in high yields as a mixed sulphide typically containing about 55% Ni and 5% Co, i.e. with a Ni:Co ratio of about 10:1. This process is expected to become increasingly important economically in the future as the nickel mining industry switches to the treatment of laterite ores, as economically viable sulphide ore reserves are depleted. Material of this composition cannot be treated economically by the existing hydrometallurgical processes described supra.
Several methods of refining mixed nickel-cobalt sulphides ofthis type have been proposed in recent years, although none is known to have been commercialized successfully. A process described in Chemical Engineering, Sep. 7, 1959, page 145, included an acidic pressure leach to oxidize nickel and cobalt sulphides to the corresponding sulphates, solution purification to remove iron, aluminium, copper, lead and zinc, and selective reduction of nickel with hydrogen. However, since the cobalt content of the solution fed to the nickel reduction operation was about 5 g/L, excessive contamination of the nickel powder with cobalt was unavoidable, and the nickel powder would not meet current market cobalt specifications. However, during recent years hydrometallurgical processes dependant upon acidic pressure leaching techniques have been using solvent extraction to effect cobalt-nickel separation.
A number of methods proposed for the separation of cobalt from nickel have been based on the differential solubilities of the ammine complexes of nickel (II) and cobalt (III). Such methods have been proposed, for example, in U.S. Pat. No 2,728,636 to Van Hare; U.S. Pat. No. 3,227,513, to Alexa et al.; U.S. Pat. No. 3,967,957, to Fonseca and U.S. Pat. No. 4,208,380 to Hamalainen.
Prior art processes for the separate recovery of nickel and cobalt from sulphuric acid leach liquors are exemplified by South African Patent No. 75 00026. This patent describes a process in which a minor portion of the feed solution containing nickel and cobalt is neutralized with ammonium hydroxide to precipitate nickel (II) hydroxide, which is then oxidized to Ni (III) hydroxide, using a strong oxidant, such as a hypochlorite or a persulphate. This nickel (III) hydroxide is then used as a reagent to precipitate cobalt (III) hydroxide selectively from the major portion of the feed solution, to produce a nickel solution depleted in cobalt. The precipitate, which is a mixture of nickel (II) and cobalt (III) hydroxides, is further treated by redissolution in ammoniacal ammonium sulphate to form a solution containing nickel (II) and cobalt (III) pentammine sulphates, from which nickel is selectively precipitated as crystalline nickel ammonium sulphate, by acidifying the solution with sulphuric acid. The final traces of nickel are removed from the purified solution by ion exchange.
Kerfoot, in U.S. Pat. No. 5,468,281, broadly teaches a process for producing cobalt powder from nickel-cobalt sulphides which involves precipitating the triple salt of cobalt (III) hexammine sulphate, nickel (II) hexammine sulphate and ammonium sulphate. More specifically, the nickel-cobalt sulphides are pressure leached under oxidizing conditions in an ammoniacal ammonium sulphate solution at a temperature of at least 80.degree. C. at an effective ammonia to metals molar ratio ranging between 5:1 to 6.5:1 to oxidize the nickel and cobalt sulphides to sulphate thereby producing an ammoniacal leach liquor in which dissolved cobalt is predominantly in the (III) oxidation state, and a leach residue. The leach liquor is separated from the leach residue which is washed and discarded. The leach liquor is then saturated with an effective amount of anhydrous ammonia and cooled to below 50.degree. C. to thereby precipitate the triple salt comprising cobalt (III) hexammine sulphate, nickel (II) hexammine sulphate and ammonium sulphate. The cobalt-depleted leach liquor is passed to a nickel recovery circuit. The precipitated triple salt is recovered from the leach liquor and repulped with water to selectively leach nickel (II) hexammine sulphate and to produce a crystalline cobaltic (III) hexammine sulphate having a Co:Ni ratio of at least 100:1 and a nickel enriched leach liquor. The cobaltic (III) hexammine sulphate is recovered, dissolved in ammonium sulphate solution and ammoniated to precipitate recrystallized cobaltic (III) hexammine sulphate having a Co:Ni ratio of at least 1000:1, which is subsequently treated to produce cobalt powder therefrom.
Deleteriously, however, using the process of the '281 patent to separate cobalt from an ammoniacal ammonium sulphate leach liquor results in a significant dilution of the nickel-rich solution thereby reducing the productivity of the subsequent nickel metal process steps. A further economic disadvantage to this process is that it produces a nickel solution containing an elevated level of free ammonia which must be removed by distillation before nickel can be recovered using conventional processes.